Effect of Adsorbents on Resistance Parameters of Heavy Metal-contaminated Clayey Sand Soils

Abstract

Combining soil with some adsorbents improves soil structure and shear strength. Thus, an optimal ratio of some adsorbents in the soil composition enhances soil adsorption capacity, reduces the possibility of groundwater contamination with these hazardous compounds, and leads to increase soil resistance parameters. This paper investigates the effect of heavy metals and adsorbents on the geotechnical behavior of clayey sand soils contaminated with lead and zinc as heavy metals as well as zeolite and rice husk ash as adsorbents. Clayey sand (mixture of sand with 20% kaolinite or bentonite) was considered as a base composition and the behavior of it was studied in both contaminated and uncontaminated states. Then, for increasing the heavy metal adsorption capacity of clayey sand, two types of adsorbents (zeolite and rice husk ash ) were added to the base composition and their behavior were investigated in the case of Lead and Zinc contamination. The results revealed that replacing the rice husk ash and zeolite adsorbents in the sand combination with 20% kaolinite clay significantly reduced the concentration of lead and zinc nitrate in the solution. Replacing 15% of rice husk ash with kaolinite heightened the absorption of lead nitrate and zinc nitrate by 228.8% and 291.6% in kaolinite sand. It was also found that adding nitrate to kaolinite increased the liquid and plastic limits. According to the results, in kaolinite, the value of the liquid limit rose from 49.8 to 59.1 upon elevating the concentration of lead to 5000 ppm, while the plastic limit also increased from 31 in the non-contaminated state to 36.4 in 5000 ppm in the infected state. According to obtained results, the dispersed structure is formed by increasing the concentration of lead nitrate in composition of sandy clay with low plasticity adsorbent; thus, shear resistance decreased. Changing in type of clay minerals to high plasticity cause the different trend in shear resistance parameters. Increasing the concentration of lead nitrate in bentonite composition, lead to flocculated structure be formed and shear strength increased.